In a gas turbine engine, there are fluid pressure variations between axially adjacent zones, such as adjacent zones through which the turbine shaft passes, with resulting leakage of fluid, e.g., air and/or other gases, between the zones. In particular, there is typically leakage at clearances between stationary and rotating parts of a turbine engine wherein a leakage flow occurs from a higher pressure zone to a lower pressure zone across the clearance between the rotating part and the stationary part. In order to improve the thermodynamic efficiency of the engine, the leakage flow needs to reduced or minimized, such as by means of a seal provided in the annular space between the two relatively moving parts.
Generally, two types of seal are commonly used in gas turbine engines to reduce leakage between components, and in particular to reduce leakage that may occur at the rotating shaft. These seals comprise a brush seal and a leaf seal. Brush seals typically comprise a plurality of fine bristles that are held in a carrier mounted on a housing wherein the tips of the bristles wipe against the rotating shaft. Brush seals have been found to be substantially effective, providing satisfactory sealing during initial use, but experience deteriorating performance after an extended period of performance. The deterioration of performance may be due to various factors including increasing brittleness of the bristles over time and insufficient rigidity to resist flexing in the direction of a pressure gradient between axially adjacent zones, particularly in the presence of high pressure gradients.
Leaf seals generally comprise a plurality of seal strips mounted to a carrier member and packed closely together in the circumferential direction. The flexible strips may bend in the circumferential direction, but exhibit a high resistance to bending in the axial direction, ensuring that the flexible strips will continue to wipe against the rotating shaft even in the presence of high pressure differentials between the axially adjacent zones. It is believed that the seal strips of leaf seals have greater durability than the bristles of brush seals and therefore provide a longer service life.